Ceramic resistive-type temperature sensors have been used widely for low cost temperature sensing. The operating temperature range of the temperature sensors currently available, however, is very limited. Conventional ceramic temperature sensors operate at temperatures less than 400 degrees Centigrade (C.). In certain applications, for example in self-cleaning cooking ranges, the required operating temperature can range as high as 600 degrees Centigrade.
Several general types of temperature sensors have been disclosed in the prior art. For example, Japanese Application No. J53-138096 discloses a thermistor comprising a solid solution of magnesium, aluminum, chromium and iron oxides as a principal component and oxides or carbonates of nickel, cobalt, zinc, titanium, barium and lanthanum as additives. A preferred composition includes Mg(Al.sub.x Cr.sub.y Fe.sub.z).sub.2 O.sub.4 as the solid solution wherein x+y+z=1. Additives including La.sub.2 O.sub.3 and TiO.sub.2 may also be present.
Japanese Application No. J53-107696 discloses a thermistor comprising a sintered mixture prepared from the following materials: La.sub.2 O.sub.3, CrO.sub.3, SnO, TiO.sub.2, Cu.sub.2 O, CaCO.sub.3, Bi.sub.2 O.sub.3, NaHCO.sub.3, SiO.sub.2 and Al.sub.2 O.sub.3. The principal components of the mixture include lanthanum, chromium and tin. Each component of the mixture, however, is employed as a base material; a dopant is not used
Russian Patent No. SU-995130 discloses a thermoresistive material containing lanthanum oxide (La.sub.2 O.sub.3), aluminum oxide (Al.sub.2 O.sub.3) and an additive selected from (1) chromium oxide, copper oxide or vanadium oxide; (2) a mixture of chromium and copper oxides; or (3) a mixture of chromium, copper and vanadium oxides. One composition is a lanthanum aluminate, with chromium being present only as a dopant at a concentration of two percent.
German Application No 2,605,804 discloses thermistor compositions as sintered mixtures of magnesium aluminate, magnesium chromate and lanthanum chromite. As illustrated in a phase diagram (FIG. 2) of that publication, a composition containing primarily lanthanum chromite is outside the scope of the compositions disclosed in that publication.
Platinum-based resistive sensors, however, are the only currently available means for sensing temperatures up to 600 degrees Centigrade. Such devices are relatively expensive and have a low sensitivity. In order to reduce the cost of the sensor and to enhance the sensitivity, a medium temperature range, electrical resistive ceramic sensor is needed.